The present invention relates to a magnetic refrigerator which produces refrigeration by applying and removing a magnetic field to a working substance for magnetic refrigeration, and particularly to a rotary magnetic refrigerator adapted to produce superfluid helium.
As is well known, the principle of the magnetic refrigerator is based upon the magnetocloric effect of a magnetic material which generates heat when a magnetic field is applied thereto, and which absorbs heat when the magnetic field is removed therefrom. Superfluid helium can be produced by magnetic refrigeration, i.e., by using a rotary magnetic refrigerator. The rotary magnetic refrigerator is effective for solving the problems inherent in the existing rotary and reciprocal refrigerators which have so far been announced as magnetic refrigerators for producing superfluid helium. They, however, still do not utilize effectively the intense magnetic fields established by electromagnets. For instance, the rotary refrigerator disclosed in U.S. Pat. No. 4,033,734 is of a wheel type in which magnetic material is arranged over the entire outer peripheral surface of a pillar. According to this refrigerator, however, when a magnetic material which has a very good heat conductivity, such as gadolinium gallium garnet (Gd.sub.3 Ga.sub.5 O.sub.12) is used as the working substance, there are large thermal losses and the efficiency is low because of the heat conduction of the working substance in the circumferential direction.
The refrigerator of the rotary wheel type developed by W. A. Steyert (J. Appl. Phys. 49(3), 1978), on the other hand, has a problem with regard to maintaining seals, since a working fluid is allowed to flow into a rotor. It is also necessary to provide means for circulating the fluid, making the equipment very complex.
In a refrigerator of a reciprocating type in which a magnetic material is reciprocally inserted into an intense magnetic field and is removed therefrom, proposed by R. Beranger et al. (Advances in Cryogenic Engineering, Vol. 27, p. 703, 1982, Plenum Press, New York), a mechanism is needed to convert rotary motion into linear motion, when it is driven by a motor. In addition, when the stroke is large, the construction of the drive portion must be complicated and large.